CHICAGO, Oct 2 (Reuters) - U.S. corn and soybean futures firmed for a second day on Tuesday on expectations for improved demand following a recent price slump and as a rainy weather outlook for the Midwest threatened to stall harvesting.

Futures jumped sharply on Monday on news of a newly negotiated U.S.-Mexico-Canada Agreement (USMCA), a replacement for the North American Free Trade Agreement, offsetting pressure from big supplies and forecasts for a bumper harvest.

Corn and soy drew additional support from a surging wheat market, which reversed early losses and jumped more than 2 percent on worries about a possible suspension of grain loading in two Russian export regions.

“I think grain demand bulls are taking positions here, given the new USMCA, and given rain delays for row-crop harvests,” said Mike Zuzolo, president of Global Commodity Analytics.

The U.S. Department of Agriculture said the corn and soybean harvests remained ahead of the normal pace as of Sunday, at 26 and 23 percent complete, but rain in key production areas of the Midwest is poised to stall harvest progress.

“The 11- to 20-day outlook has been kicking up more rainfall totals and some concerns about flooding in the western Corn Belt,” Zuzolo said.

Chicago Board of Trade December corn ended 1-3/4 cents higher at $3.67-1/2 per bushel, a three-week high. November soybeans gained 8-1/4 cents to a 5-1/2 week peak of $8.66 a bushel.

Both markets, however, remain anchored by ample U.S. and global supplies.

INTL FCStone on Monday raised its estimate of the average U.S. 2018 corn yield to 182.7 bushels per acre (bpa), from 177.7 bpa in August. The brokerage raised its forecast of the U.S. 2018 soybean yield to 54.0 bpa, up from 53.8 bpa.

Brazil’s 2018-19 soybean crop is seen reaching a record 120.4 million tonnes, a Reuters poll found.

The USDA on Friday reported higher-than-expected quarterly stocks of U.S. grains and soybeans.

Wheat futures reversed early losses on concerns about a possible suspension of grain loading in Russia, the world’s top wheat exporter.

Russia’s agriculture safety watchdog warned that 30 grain loading points in two export regions could be temporarily suspended for up to 90 days for violations of phytosanitary rules.

CBOT December wheat ended up 9-3/4 cents at a one-week high of $5.19-1/4 a bushel. (Additional reporting by Michael Hogan in Hamburg and Naveen Thukral in Singapore, editing by Tom Brown and Chizu Nomiyama)